Recently, much interest has been directed to an optical recording system which uses a method of optically recording data on a film with an He-Ne laser, Ar laser or semiconductor laser, and then reproducing the data from the film. This system permits high speed access to data.
In the optical recording system, data are recorded as follows: a laser beam is focused on a film-like recording medium to locally increase the temperature in order to change optical characteristics (such as reflection factor, transmittance and refractive index) of the recording layer, or to melt, deform, evaporate or remove the recording layer, thereby permitting data to be written on the recording medium.
The optical recording medium comprises a substrate and a recording layer. The substrate is in general, made of glass, polycarbonate, epoxy resin, polymethylmethacrylate, polyacrylonitrile, or polypentene. The recording layer is a thin film formed by vacuum deposition or sputtering, which contains one or more of the following elements: Te, Sn, In, Au, Ag, Pt, Se, As, Bi, Al, Ga, Ge, Mo, Pb, Cu, Sn, Zn, Ni, Ti, Sb and B.
If the recording layer is completely exposed, then it is oxidized by oxygen in the air, or deteriorated by contact with dust or the human body. Even if the recording layer is not exposed, it may be deteriorated by water molecules from the substrate surface or a small quantity of gas which is formed inside the substrate and comes out through the surface. A two-sided recording medium is relatively free from the above-described difficulties. However, since it is formed by bonding two symmetrical recording media together with adhesive (thermo-setting resin or radiation-setting resin), it is also deteriorated by monomers or reactive low molecular weight compounds which are produced by the adhesive. This is a serious problem for the two-sided recording medium. In order to overcome these difficulties, a method has been proposed in the art in which the upper surface and/or the lower surface of the recording layer is covered with a protective film such as an ultraviolet-setting protective layer (Japanese Patent Application (OPI) No. 169238/1981 (the term "OPI" as used herein means "an unexamined published application").
However, the conventional method is still disadvantageous in that the protective film is an insufficient barrier against water, low molecular weight compounds and ions. Thus, deterioration with time of the recording layer when held at high temperature and high humidity cannot be completely eliminated.